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Fig. 4. Variations of Ca
2þ
concentrations and calcite supersaturation index (SI
Calcite
) at the Westerh¨ fer Bach
dependent on run-off. Higher water run-off (August 2007: c.6-7Ls
21
) results in higher Ca
2þ
concentrations due to
dissolution of evaporites in the catchment area, compared to times of lower run-off (May 2006: c.3-4Ls
21
; March
2007: c.2Ls
21
). SI
Calcite
reaches maximum values later during the course of the stream at times of higher run-off.
rising pH, and while HCO
3
2
, which is several
orders of magnitude higher in concentration,
remains virtually constant, CO
22
activity has
to increase in the bulk water. At constant
Ca
2þ
then rising CO
22
steadily increases the
ion activity product of calcium carbonate
and supersaturation of the stream water with
respect to calcite. There is no detectable
impact of photosynthesis on bulk water
hydrochemistry of the investigated karstwater
streams.
(3)
From comparison of the four investigated
stream data, the maximum saturation level
does not depend on the initial Ca
2þ
to alka-
linity ratio. Depending on the charge balance
(i.e. via ions other than Ca
2þ
, HCO
3
2
and
CO
22
) streams may start degassing at a differ-
ent Ca
2þ
/alkalinity ratio, but will reach the
same saturation state (around SI
Calcite
¼ 1.0)
necessary to start significant precipitation.
Thereby, those waters with initially lower
Ca
2þ
concentration and higher alkalinity will
be higher in pH and lower in PCO
2
. On the
other hand, contribution of foreign ions with
inhibiting effect on calcite precipitation, such
as Mg
2þ
and SO
22
(Reddy 1986; Liu et al.
1995), are the likely cause of shifting the
maximum saturation level above 10-fold.
Such a foreign ion effect is suggested to
explain the higher maximum saturation index
of 1.15-1.20 in case of the Westerh¨fer Bach,
Deinschwanger Bach, and Reinsgraben
which are higher in Mg
2þ
and SO
22
concen-
trations. Instead, in the Erasbach rivulet, where
Mg
2þ
and SO
22
concentrations are negligible,
maximum saturation index is 0.9.
(2)
Due to a nucleation barrier (De Yoreo &
Vekilov 2003), calcite precipitation does
obviously not commence significantly until
a supersaturation of about 10-fold (SI
Calcite
of 0.9 to 1.1) is reached, similar to several
other field investigations of karstwater
streams (e.g. Dandurand et al. 1982; Suarez
1983; Herman & Lorah 1987, 1988; Liu
et al. 1995; Merz-Preiß & Riding 1999;
Kano et al. 2003; Kawai et al. 2006). In the
downstream sections, calcite precipitation
is evident from decreasing Ca
2þ
concen-
trations and total alkalinity. Precipitation pro-
duces an equivalent amount of CO
2
that
simultaneously escapes from the water. At
comparable rates of precipitation and degas-
sing then, pH, calcite saturation, and PCO
2
keep nearly constant as observed in the lower-
most investigated downstream reaches of
the streams, where waters were yet not in
equilibrium with atmospheric PCO
2
.
Microorganisms
Cyanobacteria, diatoms and non-photrophic pro-
karyotes, all potentially involved in CaCO
3
min-
eral nucleation via exopolymers and/or alterating
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